1. Technical Field
The present invention relates to a system of a wireless local area network (LAN) and, more specifically, to a system of a wireless LAN based on transmit power control, and to a method for controlling transmit power in a wireless LAN. In accordance with the invention, the transmit power control is performed between an access point (AP), which is the base of a wireless LAN system based on transmit power control, and a station so that stations that are outside a basic service set (BSS) area can be associated with the BSS.
2. Related Art
A local area network (LAN) can, in general, be divided into a wired LAN and a wireless LAN. The particular type is determined according to whether or not there is a cable.
The wireless LAN is a scheme for performing communication in a network using a radio wave rather than cable. The wireless LAN emerged as an alternative for solving difficulties of installation, maintenance and movement of communication equipment due to cabling, and its use is on the increase since the number of mobile users is increasing.
A wireless LAN comprises an access point (AP) and a wireless LAN card. The AP is a device that sends a radio wave to enable wireless LAN users within the transmission distance or range to have access to the Internet or to perform networking, and it acts as a base station for a mobile phone or a hub of a wired network. An AP device is also installed in the service area of a wireless ultra high-speed Internet service provided by an Internet Service Provider (ISP).
The user has to install a wireless LAN card in a station, such as a notebook computer (PC) or a PDA, in order to perform wireless network communication. Such a wireless LAN station is normally referred to, and is hereinafter referred to, as a station (STA).
IEEE 802.11 sets forth a currently widely used wireless LAN standard: “Standard for Information Technology-Telecommunications and Information Exchange between Systems-Local and Metropolitan Area Networks-Specific Requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” (1999 Edition).
The IEEE 802.11 standard defines a rule for a physical layer and a medium access control (MAC) layer which make up the wireless LAN.
The MAC layer defines an order and a regulation that should be observed when a station or an apparatus that uses a shared medium uses a medium or has access to the medium, and enables the station or apparatus to efficiently use the capacity of the medium.
A wireless LAN based on the 802.11 MAC and the IEEE 802.11a/b/g PHY has spread to homes and offices, and has become quite popular.
The IEEE 802.11 standard contentiously occupies a wireless channel to provide communication using a carrier sense multiple access with collision avoidance (CSMA/CA) scheme. In this regard, the intensity of the transmitting radio wave which uses the wireless channel is permitted when it satisfies the regulatory maximum transmit power which each nation regulates. As to Korea, for example, each channel cannot exceed 200 mW. There are also regulations for each frequency in the United States and in the European countries.
Transmit power of an AP and transmit power of stations are set in accordance with respective regulatory transmit power maximums. In Europe, however, it is compulsory that a transmit power control (TPC) function and a dynamic frequency selection (DFS) function be used when the IEEE 802.11a standard is used on a band of 5 GHz in order to protect radar or communication satellites, and IEEE 802.11h TG functions to establish such technical regulation.
TPC is a technology that protects radar or communication satellites by controlling the intensity of the transmission power so that only a required amount of power is used. DFS is a technology that changes the channel of the WLAN to another frequency band in order to protect the existing system, most preferably in cases where a radar signal or a communication satellite signal is detected.
Besides providing the advantage of protecting radar and communication satellites, TPC provides another advantage in that it can control the output of the AP and dynamically change the service area of the BSS so that electronic wave interference between BSSs can be reduced, and electronic resources can be efficiently controlled. In addition, power consumption of the station can be reduced so that batteries can be used for a longer period of time.
The wireless LAN system having transmit power control can operate in three transmit power ranges: regulatory maximum transmit power (RMTP), local maximum transmit power (LMTP), and current transmit power (CTP).
LMTP is the maximum transmit power used in the BSS, and it is different from RMTP, which is nationally regulated.
LMTP is set to a value less than that of RMTP. It is used to protect another radio wave sharing system by limiting the maximum output of the BSS. In reality, the current transmit power (CTP) used in the AP and in the station is set to a value equal to or less than the LMTP.
Transmit power control methods proposed in IEEE 802.11h/D3.0 can be divided into a method using a beacon and a probe response and a method using a TPC request and a TPC report.
The transmit power control method using a beacon and a probe response will be explained first.
The IEEE 802.11h/D3 has several elements in addition to a beacon frame body and a probe response frame body, as defined in the 802.11 MAC, in order to realize TPC and DFS.
A country element enables the WLAN to roam internationally by providing each country with automatic knowledge of a regulation of transmit power in each frequency band through the beacon frame. A power constraint element is used to specify the LMTP, and its expression is as follows:LMTP=RMTP−Power Constraint
Accordingly, a station can recognize the maximum transmit power to be transmitted in the BSS using a beacon frame received from the AP. A TPC report element is an element for recording information on transmit power used by the AP when the AP transmits the beacon frame. The station realizes how much transmit power is needed when the station transmits the beacon frame to the AP using the TPC report element received from the AP.
That is, path loss in a channel can be calculated by comparing the reception power of the signal received by the station with the transmit power on the beacon frame recorded by the AP, and the power that the station transmits to the AP can be determined based on the path loss.
Path loss can be expressed by the following mathematical expression:Path loss of a channel=Transmit power of AP−Reception power of beacon frame received from station
Next, a method for controlling transmit power using a TPC request frame and a TPC report frame will be explained.
When a station moves in the BSS, each station has to update transmit power. The stations can update transmit power using the beacon frame that is periodically received from the AP. However, the AP has to use another kind of mechanism since it cannot update transmit power.
The AP requests a station to give updated information by transmitting a TPC request frame, and the station calculates the reception power using the received TPC request frame, and then transmits link margin information in a current position and power information that is transmitted to the AP by the station using the TPC report frame.
In the latter regard, the link margin is the difference between the reception power and the minimum reception power required for a given data rate, which is expressed as follows:Link margin=Reception power of TPC request frame−Minimum reception power required for data rate of TPC request frame
The AP can obtain information from various stations in the BSS through the TPC report frame, and all stations can receive the frame transmitted by the AP by determining the transmit power on the basis of the station having the minimum link margin or the station having the minimum reception power of the TPC report frame among the various stations.
The TPC request frame has a Category, an Action, a Dialog Token, and a TPC Request element, and the TPC report frame has a Category, an Action, a Dialog Token, and a TPC Report element.
The WLAN system, to which such transmit power control is applied, can reduce interference with radar or a communication satellite which shares a frequency in the 5 GHz band, in comparison to the WLAN system to which the transmit power control is not applied, and interferes with another BSS to a small extent when the BSS lies adjacent to the WLAN system. Also, there are advantages in that the usage efficiency of electronic wave resources may increase by reducing a radius of the BSS, and the usage time of a battery may increase by reducing the power consumption of the station.
However, since the IEEE 802.11 WLAN tries to approach the medium using the CSMA/CA method, there are defects that necessarily occur in the WLAN system to which the TPC is applied. One of them relates to a hidden node.
Stations (STA) can communicate with the AP, but they cannot communicate with each other. Accordingly, many collisions in channel contention may occur since each STA cannot properly recognize whether or not the medium is occupied. Also, a problem occurs even when the STA wishes to make an association with the BSS. In the case where a STA existing within a current transmit power range of the AP wishes to make an association, there is no problem since the corresponding STA can communicate with the AP itself. However, in the case where the STA exists in the LMTP of the AP but is outside the current transmit power coverage, a problem occurs because an advance procedure for the association cannot be properly performed.
While a STA has to recognize information on the BSS in order to make an association with the BSS, the STA cannot properly receive a beacon frame transmitted from the AP when the STA exists in the LMTP range but is outside the current transmit power range. Even though the STA recognizes the information as to the BSS, there is a problem in that it is difficult to form a link for communication since the AP does not recognize information as to the STA.
The following patents are considered to be generally pertinent to the present invention, but are burdened by the disadvantages set forth above: U.S. Pat. No. 6,567,416 to Chuah, entitled METHOD FOR ACCESS CONTROL IN A MULTIPLE ACCESS SYSTEM FOR COMMUNICATIONS NETWORKS, issued on May 20, 2003; U.S. Pat. No. 6,377,548 to Chuah, entitled METHOD FOR ADMITTING NEW CONNECTIONS BASED ON MEASURED QUANTITIES IN A MULTIPLE ACCESS SYSTEM FOR COMMUNICATIONS NETWORKS, issued on Apr. 23,2002; U.S. Pat. No. 6,285,665 to Chuah, entitled METHOD FOR ESTABLISHMENT OF THE POWER LEVEL FOR UPLINK DATA TRANSMISSION IN A MULTIPLE ACCESS SYSTEM FOR COMMUNICATIONS NETWORKS, issued on Sep. 4, 2001; U.S. Pat. No. 6,469,991 to Chuah, entitled METHOD FOR OVERLOAD CONTROL IN A MULTIPLE ACCESS SYSTEM FOR COMMUNICATION NETWORKS, issued on Oct. 22, 2002; U.S. Pat. No. 5,844,900 to Hong et al., entitled METHOD AND APPARATUS FOR OPTIMIZING A MEDIUM ACCESS CONTROL PROTOCOL, issued on Dec. 1, 1998; and U.S. Pat. No. 6,226,277 to Chuah, entitled METHOD FOR ADMITTING NEW CONNECTIONS BASED ON USAGE PRIORITIES IN A MULTIPLE ACCESS SYSTEM FOR COMMUNICATIONS NETWORKS, issued on May 1, 2001.